1. Field of the Invention
This invention relates to a process of treating metals with a coating which is adapted for heat treatment in an uncontrolled oxidizing atmosphere without causing oxidation, decarburization and other objectionable results on the surface of the treated metals. More particularly, it relates to a process of treating metals which comprises applying to said metals, a double layer coating of which the base coating layer is contiguous with the metal surface and is capable, upon heating, of evolving a certain gas, and the over-coating layer which is applied on said base coating layer is capable, upon heating, of excluding the uncontrolled oxidizing atmosphere from the surface of the base coating layer to provide and to maintain a controlled surface atmosphere containing the gas next to the metal surface during a time interval sufficient to carry out the subsequent desired heat treatment at higher temperatures than was possible before. The process is particularly adapted for prevention of the surface decarburization of the metal which is subjected to heat treatment in an oxidizing atmosphere as well as for applications requiring the carburizing and nitriding of metal and the processes in which Zn, Al, Cr, Si, Mo, W, B, Ti, Pb, Ni, Cu, Fe, Mg, Mn, Co, Ge, Ac, Se, Zr and Sn are penetrated into base metals by diffusion.
2. Description of the Prior Art
In general, steel products, such as, steel sheets and strips, are manufactured by subjecting semi-finished steels in forms, such as, slabs, beam blanks, and billets to a heat treatment followed by rolling treatments, the heating conditions in the furnace depending upon the composition and thickness of the semi-finished steels and usually being from 1150.degree.-1350.degree.C for several hours. By such a heat treatment, a large amount of scale is formed on the surface of the treated steel, and, moreover, a loss of carbon from the steel surface results from the reaction of the carbon with oxygen in the oxidizing atmosphere to form CO and CO.sub.2, so that the resultant steel sheet and strip have a decarburized layer on the surface thereof. Although the decarburized layer is beneficial to the welding of the steel product to some extent, it is unfavorable for the production of silicon steel sheets with improved magnetic properties. Particularly in the heat treatment of silicon steel, therefore, it is very important to prevent the surface decarburization.
Various anti-decarburizing agents have been developed, some of which have found wide acceptance. Further, it is already known to apply an antioxidant coat on the surface of metal to be treated. However, neither the anti-decarburizing agent coat nor the antioxidant coat provide sufficient effects. The antioxidant coat is almost ineffective for the prevention of decarburization particularly when it is applied on silicon steel which usually has a large amount of scale adhering thereto, because the oxygen in the scale serves to decrease the carbon content at the surface.
Many processes by which the surface of a metal is impregnated with another metal by diffusion have been investigated and developed by many investigators, and some of these processes have found practical uses. Many of the practical processes are adapted for employment of steel as the base metal, and the steel is heated in contact with an element-bearing material to 800.degree.-1000.degree.C for a period of more than 10 hours in a reducing or inert gas atmosphere to cause penetration of the element into the base steel to depths up to several microns with a corresponding lowered element content. To impact increased corrosion resistance to steel, zinc and chromium are usually used as the diffusing element, the processes therefor being referred to as "sheradizing" and "chromizing" respectively. To impart increased heat resistance to steel, aluminum is used, the process therefor being referred to as "calorizing". Other elements such as Si, Mo, W, B and Ti may be used to impart improved properties to steel. To impart increased surface hardness to steel, the carburizing and nitriding treatments are employed. In the former connection, steel is heated to 800.degree.-1000.degree.C in an atmosphere of carbon monoxide, methane, ethane or propane, whereby these gases decomposed on the steel surface to provide carbon which serves to carburize the steel. In the latter connection, an iron-base alloy of special composition containing Al, Cr, V, Mn and/or Si is heated to 500.degree.-600.degree.C in an atmosphere of ammonia, to produce nitrogen by the dissociation of ammonia and which penetrates into the alloy to react therein with the alloying elements. The thus formed nitride provides the hardness.
All of these processes of modifying the surface of steel by diffusion require reducing or neutral atmospheres. As the reducing atmosphere, use may be made of hydrogen or a mixture of hydrogen and hydrogen halide, such as, HCl. Also ammonium halide and halogen are sometimes usuable. When a powdered diffusing element is heated in contact with an additive, for example NH.sub.4 Cl, and an anti-sintering agent in a prepared atmosphere, penetration of the element is caused at the surface of the steel on the basis of the following reaction path. EQU NH.sub.4 Cl .fwdarw. NH.sub.3 + HCl (gas) EQU HCl (gas) + Me (powder) .fwdarw. MeCl.sub.2 (gas) + H.sub.2 EQU meCl.sub.2 + Fe (base metal) .fwdarw. FeCl.sub.2 + Me* EQU MeCl.sub.2 + H.sub.2 .fwdarw. 2HCl + Me*
(wherein Me* represents a diffusing element atom in the nascent state).
However, the prior art process and apparatus therefor have several disadvantages, one of which is that the preparation of a controlled reducing or neutral atmosphere is very costly.
Another disadvantages is that the heat source of the furnace in which the slab or billet is subjected to a heat treatment at 1150.degree.-1350.degree.C for several hours is the combustion of fuel, such as, blast furnace gas, coke oven gas, heavy oil and mixtures thereof, and, therefore, an oxidizing atmosphere is usually produced by burning the fuel with an excess of oxygen over that needed for complete combustion of the fuel. Therefore, the heating of steel with or without a diffusion coating in such an oxidizing atmosphere causes oxidation of the coating and steel surface resulting in the formation of scale in large amounts. Attempts have been made to reduce the amount of scale by utilizing induction heating in a protective atmosphere, but the preparation of the protective atmosphere in such large volumes i.e., to fill the inside of the furnace is very expensive. using
Another disadvantage is that the siliconized steels produced by the powder pack method using silicon powder, ferrosilicon powder or silicon carbide powder and by the vapor phase method using silicon tetrachloride are porous at the surface because of the rapid penetration of silicon atoms into the base steel. Such a porous surface is characterized by a lack of sufficient resistance to corrosion, so that conventional siliconized steels is seldom used as corrosion-resistant steel, but rather as an abrasion-resistant steel in view of the increased surface hardness. Attempts have been made to form a non-porous silicon-diffused layer by suing the vapor phase method with a modification such that steels containing carbon in relatively large amounts, or carburized steels are used as the base steel. In these cases, however, the heat treatment should be carried out at a temperature higher than 1100.degree.C at which the austenitic grains in the base steel grow considerably, so that the resultant siliconized steel has inferior mechanical properties.
The stress corrosion cracking of which the mechanism of formation in steel is uncertain although various theories have so far been set up may be considered to be one type of the retarded rupture which is encountered when tension steels and stainless steels are subjected to tensile stress under special conditions. On the basis that the cracking is usually caused from the surface, attempts have been made to impart increased surface softness to such steels by utilizing surface decarburization or surface reheating treatment. However, the surface softening caused by these heat treatments is based on a decrease of the content of the alloying element effective to cause the softening, or on a modification of the crystalline structure of the steel, so that only modest increases in resistance to stress-corrosion cracking can be effected. Usually the increase is by a factor of 1.3 to 1.8.
Considering the facts that the surface softening is effective to increase the resistance to stress-corrosion cracking, and that pure metals have high resistances to the stress-corrosion cracking, the present inventors have made attempts to form a diffused layer containing more than 99% of iron at the surface of the steel having a high susceptability to the stress-corrosion cracking.